Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates

Abstract

Six novel electrochromic materials, 4,4′,4′′-nitrilotribenzoates (NTBAs, a–f), with donor–acceptor structure, were designed by combining cathodically electrochromic benzoates as a color-tuning unit and anodically electrochromic triphenylamine as a fluorescence-quenching unit to achieve multi-electrochromism and electrofluorochromism. The stereochemistry of the NTBAs was investigated by single-crystal X-ray diffraction. The NTBAs showed a strong blue emission in N,N-dimethylformamide. When the NTBAs were introduced into electrochromic devices (ECDs), all six ECDs showed multicolored electrochromism. All six ECDs displayed electrofluorochromic behavior, which was possibly caused by the monocationic radical formed from the oxidation of the triphenylamine center. The ester substituents markedly influenced the colored states, switching cyclability, coloration efficiency, and electrofluorochromic properties. The compounds with saturated alkyl substituents (a–c) showed three colored states, namely yellow (at ±2.8 V), light red (at ±3.0 V), and orange red (at ±3.6 V), relatively better cyclability, high coloration efficiency (>220 cm² C⁻¹), and good electrofluorochromic properties. The compound with the shortest chain length (a) performed best. The compound with an unsaturated alkyl substituent (d) showed poor electrochromic and electrofluorochromic properties. The compounds with aryl substituents (e and f) showed three colored states, namely yellow (at ±2.6 V), purple (at ±3.0 V), and claret (at ±3.6 V), moderate coloration efficiency (ca. 200 cm² C⁻¹), and good electrofluorochromic properties. The cyclability was markedly improved when the aryl substituents contained an electron-donating group.